The present invention relates to water-washable printing plates. More particularly, it relates to a novel photopolymerizable recording composition for water-washable printing plates which contains a water-dispersible elastomeric polymer blend.
Photopolymerizable recording compositions used for the preparation of printing plates and relief plates have been frequently described and include both liquids which can be cured to form the final plate and solids. It is especially desirable to use photopolymerizable compositions capable of producing final printing plates that are easily developed yet sufficiently hard to enable repeated use. Organic solvents or aqueous alkaline solutions have generally been used in the preparation and developing of such photopolymer plates. The developing process for such plates is complex because the preparation of the developing chemicals is complicated and gives rise to a host of problems that cannot easily be avoided. One way to avoid these problems is to use a photopolymerizable composition which is developable with plain water. There are water-developable compositions available, for example see U.S. Pat. No. 3,877,939, and these compositions help avoid potential poisoning to workers and drainage problems inherent with the use of harsh developing liquids.
Prior art water-developable photopolymerizable compositions are generally characterized by water dispersible resin coatings onto metallic plates or mixtures of a water sensitive polymer with an elastomeric rubber matrix. European patent application 0,480,335 describes a light-curable elastomeric mixture useful in relief printing plates. That composition utilizes an elastomeric binder which is a blend of a styrene-isoprene-methacrylic acid block copolymer and a styrene-isoprene-styrene block copolymer, although the examples show the use of chlorosulfonated polyethylene, polyisoprene and epoxidized polybutadiene in place of the styrene-isoprene-styrene block copolymer. The copolymer containing the methacrylic acid is made by a process which requires termination or capping of the block copolymer with diphenyl ethylene before adding acrylic monomer. Diphenyl ethylene is expensive and not readily available. This block copolymer assists in the water dispersibility of the blend but the styrene-isoprene-styrene block copolymer is not as easily water dispersible.
The present invention provides a photopolymerizable recording composition containing a water-dispersible elastomeric polymer blend which is a natural result of the polymerization and the processing of the elastomer, can be made in one polymerization, and which is more easily water dispersible than the blend of EP 0,480,335.